1. Field of the Invention
This invention generally relates to a road surface friction coefficient estimating apparatus for a vehicle. More specifically, the present invention relates to a road surface friction coefficient estimating apparatus for use when the vehicle is traveling at a steady state, i.e., no acceleration or deceleration occurring in the vehicle.
2. Background Information
It is well known that a road surface friction coefficient is useful control information in anti-skid control systems to avoid locking the wheels during braking, and in traction control systems to prevent wheel slippage caused by driving torque exerted during acceleration. Therefore, it is desirable to accurately estimate the road surface friction coefficient of the road on which a vehicle is traveling.
Consequently, various road surface friction coefficient estimating methods have been proposed for use in the anti-skid control systems and traction control systems that are mounted on vehicles. Generally, such methods estimate the road surface friction coefficient by utilizing the change in vehicle velocity. For example, a common method used in anti-skid systems is to estimate the vehicle velocity while the anti-skid control system is operating, and then estimate the road surface friction coefficient based on the amount of change in the vehicle velocity. Such method is disclosed in Japanese Laid-Open Patent Publication No. 5-131912.
Similarly, in the case of traction control systems, vehicle velocity is estimated by detecting the wheel velocity from non-driving wheels while the traction control system is operating. Then, the road surface friction coefficient is estimated based on the amount of change in the vehicle velocity.
Meanwhile, in Japanese Laid-Open Patent Publication No. 11-48939, the applicants of the present invention have proposed an anti-skid control apparatus, in which the road surface friction coefficient is estimated using the driving torque or the braking torque on the wheels, the wheel load, and the rotational acceleration of the wheels during braking, based on the rotational motion equations of the wheels.
There has also been proposed another method of estimating the road surface friction coefficient which uses the driving torque or the braking torque on the wheels, the wheel load, and the rotational acceleration of the wheels, based on the rotational motion equations of the wheels. In this method, four points representing four wheels of the vehicle are plotted in a two-dimensional coordinates having an axis of driving/braking force per unit wheel load and an axis of wheel velocity. In this coordinate, a regression line can be drawn based on the points. The slope angle of the regression line with respect to the wheel velocity axis expresses the driving stiffness, from which the road surface friction coefficient can be estimated. This method is disclosed in Japanese Laid-Open Patent Publication No. 10-35443. Such method improves the anti-skid control performance and traction control performance.
These road surface friction coefficient estimating methods have proven to be useful in providing control information for an anti-skid control system and/or a traction control system. However, the above-mentioned road surface friction coefficient estimating methods still have room for improvement in view of the following observations.
The aforementioned road surface friction estimating methods generate driving forces or braking forces on the wheels in order to estimate the road surface friction coefficient. In other words, in the case of road surface friction coefficient estimating apparatuses used in the aforementioned anti-skid control systems and traction control systems, it is difficult to obtain an accurate estimate unless driving forces or braking forces are generated on the wheels. Estimation is particularly difficult when the vehicle is traveling in a steady state, i.e., neither accelerating nor decelerating.
Furthermore, in the aforementioned road surface friction coefficient estimating apparatuses, when driving forces or braking forces are generated on the wheels in order to estimate the road surface friction coefficient during steady-velocity travel, the resulting acceleration or deceleration of the vehicle causes awkwardness in the driving.
Also, some of such road surface friction coefficient estimating apparatuses require a sensor, such as a small wheel-shaped sensor, in addition to the regular wheels of the vehicle, such that the small wheel-shaped sensor contacts the road surface. In that case, the price of the vehicle will increase due to the cost of the sensor. Additionally, there will be a need to decide where the sensor should be mounted.
Accordingly, the existing road surface friction coefficient estimating methods and apparatuses do not have the ability to estimate the road surface friction coefficient accurately and easily while the vehicle is traveling in a steady state, without causing an awkwardness in driving due to the acceleration or deceleration of the vehicle which is caused for the purpose of estimation of the road surface friction coefficient. Additionally, the aforesaid functions of road surface friction estimation should be preferably provided in a simple manner.
This need for accurate and easy road surface friction coefficient estimation method and apparatus that function while the vehicle is traveling in a steady state is further emphasized in view of various vehicle control systems and vehicle stabilizing apparatuses which are likely to be introduced in vehicles in the near future. These vehicle control systems and vehicle stabilizing apparatuses automatically generate braking forces in a vehicle in various manners in order to avoid obstacles by using information provided by external sensors. In these systems, the distance between the vehicle and the obstacle and relative velocity are detected by the external sensors. Automatic braking is executed in order to avoid the obstacle. Particularly in these systems, it is inherently necessary to be able to operate appropriately whenever obstacle avoidance is necessary, even when the vehicle is traveling in a steady state. If the road surface friction coefficient cannot be estimated accurately, the automatic braking may not be able to generate sufficient braking force. Therefore, the road surface friction coefficient estimation is indispensable in these vehicle control systems.
In view of the above, there exists a need for a road surface friction coefficient estimating apparatus and methods which overcome the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
An object of the present invention is to provide a road surface friction coefficient estimating apparatus that can function while the vehicle is traveling in a steady state.
Another object of the present invention is to provide a road surface friction coefficient estimating method that can function while the vehicle is traveling in a steady state.
This object is basically attained by providing a road surface friction coefficient estimating apparatus for a vehicle equipped with a plurality of wheels. The road surface friction coefficient estimating apparatus has target driving/braking force setting means, driving force generating means, braking force generating means, wheel velocity detecting means, wheel load detecting means, and road surface friction coefficient calculating means. The target driving/braking force setting means sets a pre-selected torque to be generated. The driving force generating means generates a driving force in at least one of the wheels in accordance with the pre-selected force set by the target driving/braking force setting means. The braking force generating means generates a braking force in at least one of other wheels in accordance with the pre-selected force set by the target driving/braking force setting means. The braking force has a magnitude to cancel the driving force. The wheel velocity detecting means produces a wheel velocity signal indicative of a wheel velocity. The wheel load detecting means produces a wheel load signal indicative of a wheel load. The road surface friction coefficient calculating means calculates a road surface friction coefficient based on the wheel load, the wheel velocity, and the braking and driving forces.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.